Respiratory syncytial (RS) virus, a major cause of lower respiratory disease in infants, matures like myxo- and other paramyxoviruses by budding through the infected-cell surface enclosed in an envelope derived from the cell's plasma membrane. Several aspects of this maturation process are unknown. The proposal's author established that a complete RS-virus growth cycle in HeLa cells requires 20 to 30 hours including a 10-12 hour latent period, then synchronized the infection at a late stage by using medium with a limiting phenylalanine (PA) concentration. The cycle is arrested at a stage where all the viral RNA for a full yield of virus and at least some nucleocapsid protein have been made but no nucleocapsid moves to periphery of cells and no viral antigen appears in plasma membranes. If PA is then added the cycle resumes; in 2-3 hours complete virus is produced and viral antigen is demonstrable in plasma membranes. With this compressed latent period, the events leading to virus budding could be followed closely. Biochemical, immunological and electromicrograph studies using the synchronized infection are proposed, to determine where viral envelope proteins are made, how and in what form they are incorporated into plasma membranes and how nucleocapsid is transported. The studies would provide information on RS virus: its maturation process, how synthesis of its structural components is organized and how it compares in replication and assembly with other paramyxoviruses, from which it differs in several respects. The information gained may be applicable to the assembly and maturation of other membrane-budding viruses, such as the mumps, measles, parainfluenza, influenza, rabies, and leukemia viruses, as well as to the little understood processes of plasma synthesis and assembly.